[질문] 7의30승을 10진법으로나타내면?

7^n은7*7^n-1로고칠수있죠?그렇게해서풀면뒤에마이너스일은빼기에의해사라지구요(7*7^n-1)-(7^n-1)이되니까같은거묶어내면(7-1)(7^n-1)=6*7^n-1이되겠네요^^